scripts/travis-ci: bump apt timeout in before_install.bash.
scripts/travis-ci: bump apt timeout in before_install.bash.

After switching to using a mirror of MXE on dl.mumble.info we've begun
running into timeouts from apt on Travis CI. For example:

	E: Failed to fetch https://dl.mumble.info/mirror/pkg.mxe.cc/repos/apt/debian/pool/main/m/mxe-x86-64-unknown-linux-gnu-cmake/mxe-x86-64-unknown-linux-gnu-cmake_3.5.2-20170208_amd64.deb  Operation too slow. Less than 10 bytes/sec transferred the last 10 seconds

The error is from cURL, and it means that the options CURLOPT_LOW_SPEED_LIMIT
and CURLOPT_LOW_SPEED_TIME are set quite low.
(See https://github.com/curl/curl/blob/bb0ffcc36fd46298e02f943b255ee5d6553c06dc/lib/speedcheck.c)

To fix this, we set the Acquire::http::Timeout option to 120 seconds.
This causes apt's https transport to set CURLOPT_LOW_SPEED_TIME to 120.
(See https://github.com/Debian/apt/blob/debian/wheezy/methods/https.cc#L224-L230)

Fixes mumble-voip/mumble#3312

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble