Add testing on PHP 7 on Travis

[stof]

This is the first step for #825: running tests on PHP 7 (with failures allowed as the library is not compatible yet).
memcache.so is not available for PHP 7 on Travis currently, so enabling it is skipped (as done for HHVM currently).

I als removed a few useless sudo usages for Composer calls

[im-denisenko]

Sudo isn't useless. Ansible runs as root and owner for vendor directory is root also. There must be either chown vendor or sudo composer.

[stof]

hmm indeed.

It may be better to limit the usage of sudo on Travis to be able to use their new infrastructure though. I will revert the sudo removal.

[ruflin]

@stof @im-denisenko I'm a little bit undecided if I should merge the pull request already as it will still fail for quite some time or if we should wait until there are also the changes for PHP 7 compatibility.

@stof About the new travis infrastructure: I'm still hoping that they will support docker containers which would heavily speed up the builds.

[stof]

@ruflin regarding the change for PHP 7, the first question is to decide what the new name of these Bool classes should be (both the query and the filter). It is easy to keep BC for PHP 5.x users with a class_alias (on PHP 7, the only option is to use the new name as Bool is forbidden)

[im-denisenko]

@ruflin I tried to run tests on php 7 and got some weird errors, like segmentation fault in http transport test, which could be not related to Elastica and not be fixed soon. Hence I guess, it could be merged already.

About broken classes. They were named Filter\BoolFilter, Query\BoolQuery and Transport\NullTransport until #303. We could just rename them back, though it damages naming consistency.

[ruflin]

I merged the pull request so we know what the status is. I suggest we move all further discussions to #825