Uses head version of rvm to avoid shell_update_session not found error (#1791)

Uses head version of rvm to avoid shell_update_session not found error

Fixes #1786 

tests.sh

@@ -238,6 +238,10 @@ build_objectivec_osx() {
 build_objectivec_cocoapods_integration() {
   # First, load the RVM environment in bash, needed to update ruby.
   source ~/.rvm/scripts/rvm
+  # Update rvm to the latest version. This is needed to solve
+  # https://github.com/google/protobuf/issues/1786 and may not be needed in the
+  # future when Travis updates the default version of rvm.
+  rvm get head
   # Update ruby to 2.2.3 as the default one crashes with segmentation faults
   # when using pod.
   rvm use 2.2.3 --install --binary --fuzzy