Protobuf 3.4 and update to Serializer

[michaelbausor]

This PR updates the protobuf dependency to the soon-to-be-released 3.4, and migrates the Serializer class to use the newly released descriptor classes.

It also contains minor bugfixes required to support the protobuf c extension, which from 3.4 onwards will be side-by-side compatible with the composer package.

NOTE: Travis is expected to fail until 3.4 is actually released.

[michaelbausor]

@garrettjonesgoogle @shinfan @neozwu Hi all, gentle ping on this after fixit week

[shinfan]

As discussed offline, we will need to remove these GPBXXX classes once the protobuf ones become public in the future.

[michaelbausor]

Filed issue: #93

[shinfan]

LGTM after tests pass

[codecov-io]

Codecov Report

Merging #92 into master will increase coverage by 1.35%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage   89.16%   90.52%   +1.35%     
==========================================
  Files          32       33       +1     
  Lines        2179     2249      +70     
==========================================
+ Hits         1943     2036      +93     
+ Misses        236      213      -23

Impacted Files	Coverage Δ
src/Testing/GeneratedTest.php	100% <100%> (ø)	⬆️
src/Serializer.php	93.25% <100%> (+1.75%)	⬆️
src/GrpcCredentialsHelper.php	100% <0%> (ø)	⬆️
src/BackoffSettings.php	100% <0%> (ø)	⬆️
src/LongRunning/OperationsClient.php
src/LongRunning/OperationsGapicClient.php	90.41% <0%> (ø)
src/ValidationTrait.php	83.33% <0%> (ø)
src/Testing/MockStubTrait.php	91.42% <0%> (+4.28%)	⬆️
src/Testing/SerializationTrait.php	92.85% <0%> (+14.28%)	⬆️

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