MenuEditor extension 1.7.0: minor bugfix and italian translation

Courtesy of @iplume
Added confirmation for unlocking the slug. Might help with people inadvertently breaking links. Fixes #1364
Optimized usage of composer on travis.

There is a preinstalled version of composer on travis-ci, so we should use it.
But still keeping the `self-update`, but making it failsafe with `|| true`, if getcomposer.org might be down.
Fixed XSS in snippets generator

Under certain circumstances, parts of the client-supplied URL path would
be used in the "canonical" <link>, and no HTML-encoding was applied to
secure this, causing a reflected XSS vulnerability.

I have not been able to reproduce the exploit on my test environment,
but I have verified it on another system.
Czech translation

all string translated
Removed backticks which were causing PostgreSQL installations to crash
Bumping version to 1.6.7