Fix compiling dart::common::make_unique() with recent GCC versions

[jslee02]

Explicitly specify the namespace as ::dart::common::make_unique() to avoid the ambiguity with std::make_unique().

Note that this PR won't pass the CIs (for Ubuntu >= 14.04) until the issues of gtest and #480. I breakdown #479 into smaller ones to make the review process, and this is one of them.

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[brianhou]

My guess is that this is a problem when using a compiler that supports C++14 features -- is that correct?

[jslee02]

My guess is that this is a problem when using a compiler that supports C++14 features -- is that correct?

Exactly!