Cairo.jl fallback to source build on Travis Trusty

[yuyichao]

When trying to use Travis trusty for package testing, I notice that BinDeps failed to use apt-get to install the packages but rather fall back to compiling from source (which would be fine if it doesn't generate too much output ....)

The travis run with some debug output can be found here. The debug output that looks suspicious is

- Library "pango"
   - Satisfied by:
     - System Paths at /usr/lib/x86_64-linux-gnu/libpango-1.0.so.0
   - Providers:
     - BinDeps.AptGet package libpango1.0-0 (can't provide)
     - BinDeps.Yum package pango (can't provide)
     - Autotools Build

If I run apt-get to install all the necessary packages before building Cairo, the package build successfully. (See this travis run)

c.c. @tkelman

[tkelman]

The library group needs to get every library from the same provider. The issue is BinDeps.AptGet package libpango1.0-0 (can't provide).