Bumping number to fix ruby 2.1 on mac

* Fix more memory leak for php c extension

* Fix memory leak for php5.5

Update rake file to build of 2.1.6.

Support ruby2.5

Previously, within the same load period of protobuf module, well known
types are not initialized correctly for the second request. The reason
is that well known type are only initialized if a method local static
variable is not set. However, that variable is not reset after request
ends. Therefore, when the second request comes, the method local static
variable is still set (by previous request) and well types are not
initialized in this case.

Bump protoc-artifact version for a patch rebuild

remove nullptr

Fix uploading binary wheel.

The script now takes two prompt for user name and password. One single
input redirection no longer works.

Disable pip cache when testing uploaded packages

Otherwise, pip complains "cannot find a version satisfies the
requirement." I suspect it was due to caching issues. I usued to
manually add a `sleep 30` between the upload and the installation test
which is quite unstable.

Update changelog for 3.5.1

Add support for Windows ARM64 build

Cherrypick for csharp, including:

Fix string::back() usage in googletest.cc

Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

Replace C++11 only method std::map::at

Update version number to 3.5.1

`umask 0022` makes sure the created package will be accessible by all users.

Avoid calling method from php extension directly

Create containing directory before generating well_known_types_embed.cc

Remove py2.6 support.

available.

Use matching enum type for IsPOD.