Corrected travis image link

The old "hashbang" url redirected to https://travis-ci.org//buildout/buildout, with two slashes, which didn't work. Travis bug, but using the new https://travis-ci.org/buildout/buildout url ourselves is easier :-)
Added test demonstrating #71

`update-versions-file = true` has no effect when `show-picked-versions` is
false.
Changed 'wrote versions to' string.

I changed it to make it applicable whether the picked versions
are printed or not. 'This info has been written to' doesn't work
if this hasn't been printed on the previous lines :-)
Fixed #71: when needed, write versions to file

Picked versions are stored for later printing/writing in the Installer
also when show-picked-versions is set to false but update-versions-file is set
to a file. Previously, show-picked-versions=false would also disable the versions
file updating.
corrected 'fixe' => 'fix' typo
Merge pull request #72 from reinout/update-versions-file-without-show

Fix for #71 - looks good to me.
Merge pull request #69 from reinout/patch-4

Corrected travis image link