As a user of opencensus-cpp, I'd want dependent library locked to specific version #250

Currently, opencensus-cpp uses multiple upstream libraries, including abseil, gmock, gtest, prometheus-cpp. However, when we pulled in these dependent libraries, we pulled off from master. As a user of opencensus-cpp, if I want to build opencensus-cpp library from scratch, I may get different result from time to time.

Therefore, I suggest locking down the version for both bazel and CMake for each opencensus-cpp release.

for bazel, an example is the Bazel's absl dependent:
from

    urls = ["https://github.com/abseil/abseil-cpp/archive/master.zip"],

to

    urls = ["https://github.com/abseil/abseil-cpp/archive/20180600.zip"],

for Cmake's absl dependent, from

	GIT_TAG "master"

to

	GIT_TAG "20180600"