* Update ruby_generator.cc to allow proto2 imports in proto3, with updated unit tests

* Update Makefile.am with new ruby_generated_code_proto2_import.proto

* Fix ruby_generator unit test to use temporary test directory for imported protos

* Add test for imported proto2 to ruby/tests

* Fix proto_path, restore to ../src/protoc, and fix/cleanup unit test.

* Rename Proto2TestMessage to TestImportedMessage for consistency, for ruby compiler tests

* Add python-requires in setup.py

* Update setup.py

* Fix dist install test by ensuring that we use Python 3

Now that we have dropped Python 2 support, we need to make sure this
install test uses Python 3.

* Update Docker image to install Python 3 version of setuptools

* Run pip3 instead of pip

Performance fix for DynamicMessage: force GetRaw() to be inlined.

googletest uses a Python script in its build which is not compatible
with Python 3. Unfortunately we can't easily upgrade googletest right
now, so this commit works around the problem by putting a python symlink
pointing to Python 2 in the $PATH.

It appears that this extra conditional in travis-test.sh is no longer
necessary, and in fact we need to avoid using a custom version of
OpenSSL since that is causing its own error.

I am hoping this will fix the error we are getting as described here:
rvm/rvm#5014

Cherry-pick fixes for 3.18.1

Installing and upgrading Homebrew packages is taking quite a lot of time
(around 20-30 minutes) for each run. This commit removes all Homebrew
usage from the test runs. Homebrew may have been necessary at some point
in the past, but now it appears that everything works without it. The
preinstalled build tools seem to be sufficient for building protoc, and
Ruby is something we already get from rvm.

* Update protobuf version

* Update PHP release notes in package.xml