Stop support for PHP 5.5

[nenad]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	yes
Related tickets	no
Documentation	no
License	MIT

What's in this PR?

Stop support for PHP 5.5

Why?

Prevents us from migrating to newer version of react components.

[dbu]

i think we should bump all the way to PHP 7.1 while we are at it. and increase the minor version in the composer branch-alias so we will be 2.2. i guess in this case its very unlikely to matter, but just out of principle so that we can release security fixes that people stuck on legacy php versions.

[xabbuh]

In fact, we need to update the PHP version for the job that runs with lowest dependencies too.

[nenad]

In fact, we need to update the PHP version for the job that runs with lowest dependencies too.

I removed PHP 5.5 from the Travis build completely. Does this comment refer to adding the lowest dependencies job to PHP 7.1 or some other version?