minor #638 changed hhvm-nightly to hhvm (gondo)
minor #638 changed hhvm-nightly to hhvm (gondo)

This PR was submitted for the master branch but it was merged into the 5.x branch instead (closes #638).

Discussion
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changed hhvm-nightly to hhvm

as per https://travis-ci.org/swiftmailer/swiftmailer/jobs/70777435
`HHVM nightly is no longer supported on Ubuntu Precise. See https://github.com/travis-ci/travis-ci/issues/3788 and https://github.com/facebook/hhvm/issues/5220`

Commits
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cae8972 changed hhvm-nightly to hhvm

c960657/swiftmailer,michabbb-backup/swiftmailer,kuslahne/swiftmailer,denpamusic/swiftmailer,swiftmailer/swiftmailer,c960657/swiftmailer,zghosts/swiftmailer,Rotzbua/swiftmailer,Chhunlong/swiftmailer,deguif/swiftmailer,papagel/swiftmailer,fvosberg/swiftmailer,deguif/swiftmailer,swiftmailer/swiftmailer,gencer/swiftmailer,Rotzbua/swiftmailer,robertklein/swiftmailer,papagel/swiftmailer,kuslahne/swiftmailer,jamesggordon/swiftmailer,gencer/swiftmailer,tweakers-dev/swiftmailer,jamesggordon/swiftmailer,tweakers-dev/swiftmailer,Chhunlong/swiftmailer,robertklein/swiftmailer,voku/swiftmailer,michabbb-backup/swiftmailer,denpamusic/swiftmailer,voku/swiftmailer,fvosberg/swiftmailer,zghosts/swiftmailer