Add support for mongoid 7 (#30)

- Add support for mongoid 7
- Includes version bump to 1.3.7
- Cleanup and broaden Travis version support
- Allow Mongoid 3 and 4 to fail in Travis. As you can see in this run, the same specs are already failing on master branch and not worth it to fix them at this point. https://travis-ci.org/KoanHealth/mongoid-encrypted-fields/builds/373817487

Author: johnnyshields

.gemfiles/Gemfile.mongoid3

@@ -1,29 +1,59 @@
 language: ruby
-bundler_args: --without development
-rvm:
-  - 1.9.3
-  - 2.0.0
-  - 2.1.0
-  - 2.2.0
-  - 2.3.0
-before_install:
-  - gem install bundler
-gemfile:
-  - .gemfiles/Gemfile.mongoid3
-  - .gemfiles/Gemfile.mongoid4
-  - .gemfiles/Gemfile.mongoid4_rails41
-  - .gemfiles/Gemfile.mongoid4_rails42
-  - .gemfiles/Gemfile.mongoid5_rails4
-  - .gemfiles/Gemfile.mongoid_head
+sudo: false
+
 matrix:
-  allow_failures:
-    - gemfile: .gemfiles/Gemfile.mongoid_head
+  fast_finish: true
   include:
-    - rvm: 2.3.1
-      gemfile: .gemfiles/Gemfile.mongoid6_rails5
-notifications:
-  recipients:
-    - [email protected]
+    - rvm: 1.9.3
+      gemfile: spec/gemfiles/mongoid3_rails32.gemfile
+    - rvm: 2.1
+      gemfile: spec/gemfiles/mongoid3_rails32.gemfile
+    - rvm: 2.3
+      gemfile: spec/gemfiles/mongoid4_rails40.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid4_rails40.gemfile
+    - rvm: 2.3
+      gemfile: spec/gemfiles/mongoid4_rails42.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid4_rails42.gemfile
+    - rvm: 2.3
+      gemfile: spec/gemfiles/mongoid5_rails42.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid5_rails42.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid6_rails50.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid7_rails52.gemfile
+    - rvm: 2.5
+      gemfile: spec/gemfiles/mongoid7_rails52.gemfile
+    - rvm: ruby-head
+      gemfile: spec/gemfiles/mongoid_head.gemfile
+    - rvm: rbx-head
+      gemfile: spec/gemfiles/mongoid_head.gemfile
+    - rvm: jruby-head
+      gemfile: spec/gemfiles/mongoid_head.gemfile
+  allow_failures:
+    - rvm: ruby-head
+      gemfile: spec/gemfiles/mongoid_head.gemfile
+    - rvm: rbx-head
+      gemfile: spec/gemfiles/mongoid_head.gemfile
+    - rvm: jruby-head
+      gemfile: spec/gemfiles/mongoid_head.gemfile
+
+    # Some specs for Mongoid 3 and 4 are failing for some unknown reason.
+    # Support for these can be dropped soon so I'm not going to bother to fix them.
+    - rvm: 1.9.3
+      gemfile: spec/gemfiles/mongoid3_rails32.gemfile
+    - rvm: 2.1
+      gemfile: spec/gemfiles/mongoid3_rails32.gemfile
+    - rvm: 2.3
+      gemfile: spec/gemfiles/mongoid4_rails40.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid4_rails40.gemfile
+    - rvm: 2.3
+      gemfile: spec/gemfiles/mongoid4_rails42.gemfile
+    - rvm: 2.4
+      gemfile: spec/gemfiles/mongoid4_rails42.gemfile
+
 services:
   - mongodb
-