Also set gtest's standard to C++11

[mjcarroll]

🦟 Bug fix

Follow-up to #214

[chapulina]

It's still unhappy:

[ 44%] Building CXX object test/CMakeFiles/gtest_main.dir/gtest/src/gtest_main.cc.o
In file included from /Users/jenkins/workspace/ignition_common-ci-pr_any-homebrew-amd64/ign-common/test/gtest/src/gtest_main.cc:31:
In file included from /Users/jenkins/workspace/ignition_common-ci-pr_any-homebrew-amd64/ign-common/test/gtest/include/gtest/gtest.h:62:
In file included from /Users/jenkins/workspace/ignition_common-ci-pr_any-homebrew-amd64/ign-common/test/gtest/include/gtest/internal/gtest-internal.h:40:
/Users/jenkins/workspace/ignition_common-ci-pr_any-homebrew-amd64/ign-common/test/gtest/include/gtest/internal/gtest-port.h:836:12: error: no member named 'make_tuple' in namespace 'std'
using std::make_tuple;
      ~~~~~^

[codecov]

Codecov Report

Merging #221 (3af56b5) into ign-common3 (32fd74e) will not change coverage.
The diff coverage is n/a.

❗ Current head 3af56b5 differs from pull request most recent head 5512c08. Consider uploading reports for the commit 5512c08 to get more accurate results

@@             Coverage Diff              @@
##           ign-common3     #221   +/-   ##
============================================
  Coverage        76.56%   76.56%           
============================================
  Files               73       73           
  Lines            10357    10357           
============================================
  Hits              7930     7930           
  Misses            2427     2427           

---

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