In xcode 6.4, `brew update' breaks `brew' due to old `ruby'. (#125)

* Don't update homebrew

.travis.yml

@@ -11,6 +11,10 @@ cache:
   directories:
    - $HOME/.ccache
 
+env:
+  global:
+    - HOMEBREW_NO_AUTO_UPDATE=1
+
 matrix:
   include:
     - os: linux

tests/test_library_builders.sh

@@ -4,10 +4,6 @@ export BUILD_PREFIX="${PWD}/builds"
 rm_mkdir $BUILD_PREFIX
 source library_builders.sh
 
-# set -e -x
-
-if [ -n "$IS_OSX" ]; then brew update; fi
-
 start_spinner
 
 suppress build_openssl