buildpacks/ruby: Bump default bundler to 1.10.5

Author: fgrehm

stack/buildpacks/ruby/bin/compile

@@ -118,7 +118,7 @@ fi
 
 source ${HOME}/.profile.d/ruby.sh
 
-DEVSTEP_BUNDLER_VERSION=${DEVSTEP_BUNDLER_VERSION:-'1.8.0'}
+DEVSTEP_BUNDLER_VERSION=${DEVSTEP_BUNDLER_VERSION:-'1.10.5'}
 
 if ! $(which bundle &> /dev/null); then
   tarball_path=$2/rubies/bundler-${DEVSTEP_BUNDLER_VERSION}.gem