Fix use default ruby

Author: zapcannon87

.travis.yml

@@ -13,7 +13,7 @@ env:
     - DESTINATION="OS=11.0,name=iPhone X"          SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES"
     - DESTINATION="OS=10.3.1,name=iPhone 7 Plus"   SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES"
     - DESTINATION="OS=9.0,name=iPhone 6"           SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES"
-before_install: rvm use 2.0
+before_install: rvm use default
 install: gem install slather
 script:
   - set -o pipefail