Bump Elixir version on CI

Author: fhunleth

.circleci/config.yml

@@ -4,7 +4,7 @@ defaults: &defaults
   docker:
     - image: erlang:21.1
   environment:
-    ELIXIR_VERSION: 1.7.4-otp-21
+    ELIXIR_VERSION: 1.8.1-otp-21
     LC_ALL: C.UTF-8
 
 install_elixir: &install_elixir
@@ -32,7 +32,7 @@ install_system_deps: &install_system_deps
 version: 2
 
 jobs:
-  test_elixir_1_7_otp_21:
+  test_elixir_1_8_otp_21:
     <<: *defaults
     steps:
       - checkout
@@ -58,5 +58,5 @@ workflows:
   version: 2
   build_test:
     jobs:
-      - test_elixir_1_7_otp_21:
+      - test_elixir_1_8_otp_21:
           context: org-global