.travis.yml:remove "Tr_default" (#133)

This related script needs updating, I think. I don't know yet how best to proceed.
https://github.com/ZetaGlest/zetaglest-source/blob/develop/.travis-before_install.sh

This patch is to help fix the broken build shown at
https://travis-ci.org/ZetaGlest/zetaglest-source/builds/378943044

* remove Tr_default section

Not sure why it builds everywhere else but fails on this one.

.travis.yml

@@ -14,14 +14,6 @@ matrix:
       env: Tr_Compiler_Version="7"
       # https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test
       dist: trusty # broken compiler on 12.04
-    - os: linux
-      compiler: gcc
-      addons:
-          apt:
-            packages:
-            - libsdl2-dev
-      env: Tr_Compiler_Version="default"
-      dist: trusty
     - os: linux
       compiler: clang
       addons: