Fix build-linux. Lua51-dev not needed

Author: q4a

.travis.yml

@@ -7,6 +7,7 @@ install: export CXX="g++-7"
 os:
 - linux
 before_script:
+  - g++ --version
   - sudo apt-get install libegl1-mesa-dev libgles2-mesa-dev
   - sudo apt-get install cmake lua5.1-dev libssl-dev libogg-dev libtheora-dev libvorbis-dev libsdl2-dev liblzo2-dev libjpeg-dev libncurses5-dev
   - mkdir bin

doc/howto/build-linux.txt

@@ -2,7 +2,6 @@ TITLE: X-Ray 1.6 build and setup on Linux
 
 Required packages:
 ------
-Lua51
 OpenSSL
 Theora
 OGG
@@ -11,7 +10,7 @@ LZO
 JPEG
 
 Ubuntu 18.04 build dependencies:
-sudo apt install git cmake lua5.1-dev libssl-dev libogg-dev libtheora-dev libvorbis-dev libsdl2-dev liblzo2-dev libjpeg-dev libncurses5-dev
+sudo apt install git cmake libssl-dev libogg-dev libtheora-dev libvorbis-dev libsdl2-dev liblzo2-dev libjpeg-dev libncurses5-dev
 
 Build commands:
 ------