Update minimum php version to 7.0.0

[dbtlr]

Fixes #58

Updates the minimum version requirement to PHP >= 7.0.0.

Also fixes some of the broken unit tests in PHP 7.1 and higher. These were due to two problems:

1. The way the temp files were created to test uploaded files used tmpfile() which creates a resource, instead of a file name. fopen() no longer allows resources to be passed in. An alternative way to handle this would be to check for a resource and to use it directly, instead of calling fopen(), however we wouldn't be able to guarantee that the w flag was used. Opted to just fix how tests were written to use file names instead.
2. In URL creation empty checks were using falsy checks, which matched "0" as well. Updated to use exact !== '' checks instead.

[coveralls]

Coverage remained the same at 97.792% when pulling fadfa3f on dbtlr:update-min-php7 into 3151643 on slimphp:master.

[dbtlr]

@akrabat Could I get a review/merge on this. It could help move a few other PRs forward as well, including #27

[akrabat]

What happens if null is passed in for $password?

[akrabat]

I think we shouldn't change the specificity of the conditionals without also type hinting.

[akrabat]

Rebased due to merge of the PRs.

[akrabat]

Turns out my comments were moot!

[akrabat]

Thanks @dbtlr !