Travis fails to install a package and exits.

Once in a while I am getting this error on Travis:

```
E: Package 'cmake' has no installation candidate
E: Unable to locate package libsnappy-dev
The command "if [[ "`uname`" == "Linux" ]]; then sudo apt-get -y install ghostscript doxygen transfig vera++ cmake zlib1g-dev libsnappy-dev; fi
" failed and exited with 100 during .
```

I will add a link next time it happens.