Remove HHVM-nightly builds

[stof]

hhvm-nightly is not available anymore on Travis because HHVM dropped support for Ubuntu Precise, which is still used by Travis.

This avoids creating useless jobs in the matrix which are killed by Travis from the start.

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[Ocramius]

👍

[stof]

@Ocramius I suggest backporting this to older branches too, to avoid the useless builds

[Ocramius]

@stof will do if there is a problem with them, but I plan to reduce backporting to a minimum anyway :-)

[stof]

@Ocramius such builds are cancelled by Travis immediately after starting them. But it still requires starting them and then they appear as errored builds