[Travis] Add HHVM to test matrix
[Travis] Add HHVM to test matrix

Ignore failures until fully support of HHVM

fritsjanb/broadway,mbadolato/broadway,jaymecd/broadway,TabsterApp/broadway,broadway/broadway,tPl0ch/broadway,qandidate-labs/broadway,boekkooi/broadway,wjzijderveld/broadway