Drop GHC 7.6.3 from the build matrix

cairo-0.13.4.x seems to have a build issue with 7.6.3:
https://matrix.hackage.haskell.org/package/cairo

Author: Mitsutoshi Aoe

.travis.yml

@@ -29,9 +29,6 @@ before_cache:
 
 matrix:
   include:
-    - compiler: "ghc-7.6.3"
-    # env: TEST=--disable-tests BENCH=--disable-benchmarks
-      addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-7.6.3,libgtk2.0-dev], sources: [hvr-ghc]}}
     - compiler: "ghc-7.8.3"
     # env: TEST=--disable-tests BENCH=--disable-benchmarks
       addons: {apt: {packages: [ghc-ppa-tools,cabal-install-head,ghc-7.8.3,libgtk2.0-dev], sources: [hvr-ghc]}}

threadscope.cabal

@@ -35,8 +35,7 @@ Cabal-version:       >= 1.6
 Data-files:          threadscope.ui, threadscope.png
 Extra-source-files:  include/windows_cconv.h
                      README.md
-Tested-with:         GHC == 7.6.3,
-                     GHC == 7.8.3,
+Tested-with:         GHC == 7.8.3,
                      GHC == 7.10.2,
                      GHC == 8.0.2,
                      GHC == 8.2.1