Only attempt to install carthage if not already installed

Author: hamchapman

.travis.yml

@@ -22,7 +22,12 @@ env:
     - DESTINATION="platform=OS X,arch=x86_64"  SDK="$MACOS_SDK" SCHEME="$MACOS_SCHEME"
 before_install:
   - brew update
-  - brew install carthage
+  - if brew list | grep -q carthage; then
+      echo Carthage installed. Proceed without installing
+    else
+      echo Carthage not installed. Installing using homebrew
+      brew install carthage
+    fi
   - gem install cocoapods -v 1.0.0.beta.2
 install:
   - pod _1.0.0.beta.2_ install