Add HHVM to build matrix
minor #1294 Add HHVM to build matrix (ptarjan)

This PR was squashed before being merged into the 1.15-dev branch (closes #1294).

Discussion
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Add HHVM to build matrix

Now that HHVM can run your project, lets keep it that way

Commits
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e03b05e Add HHVM to build matrix
fixed Travis config to allow HHVM to fail