gitian: add libz-dev dependency package for linux boost #3728

Boost iostreams was picking up libz-dev in VirtualBox, as the recommended way to build is now to make a VM with all dependency packages installed.

This caused a divergence between KVM/LXC build and VirtualBox build results.

Fix this in the simplest possible way: add the libz-dev package.

Alternative to #3726, fixes a non-determinism issue in #3725.

Expected output now is (thanks @int03h):

f29b7d9577417333fb56e023c2977f5726a7c297f320b175a4108cf7cd4c2d29 boost-linux32- 1.55.0-gitian-r1.zip
88232451c4104f7eb16e469ac6474fd1231bd485687253f7b2bdf46c0781d535 boost-linux64- 1.55.0-gitian-r1.zip

As the compression part of boost::iostreams is not used by bitcoin, this should not affect the eventual build result (tested: final output is still same as mentioned in #3622).

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/6c0276ae69c6cfb9656ef7d7bf1239b299475866 for binaries and test log. This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/ Contact BlueMatt on freenode if something looks broken.

@laanwj Thank you sir.