Run sass-spec against LibSass master until 3.4 is stable

When we start shipping 3.5 betas the 3.5 target will become master

Author: xzyfer

.travis.yml

@@ -17,7 +17,7 @@ matrix:
       env: LANGUAGE_VERSION=3.5 IMPL=ruby-sass
     - gemfile: Gemfile.sass_4_0
       env: LANGUAGE_VERSION=4.0 IMPL=ruby-sass
-    - env: LANGUAGE_VERSION=3.4 IMPL=libsass COMMAND="../sassc/bin/sassc" GITISH=3.4.0-RC1
+    - env: LANGUAGE_VERSION=3.4 IMPL=libsass COMMAND="../sassc/bin/sassc" GITISH=master
     - env: LANGUAGE_VERSION=4.0 IMPL=dart-sass
 
 before_install: