Require minimum of PHP 7, enabling tests on PHP 7.1

[asgrim]

Fixes #221

Rebase when #217 is merged

[asgrim]

Oh yay there's some fun going on here.

Two issues I've spotted:

? not supported, needs PHP Parser 3 most likely needs #217 to be merged

1) BetterReflectionTest\Reflection\ReflectionParameterTest::testGetDefaultValueAsString with data set #1 ('12.3', '12.300000000000001')
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-12.300000000000001
+12.3

[asgrim]

Hrmm, I know it's not technically needed, but I think it helps; especially when php itself isn't actually semver. In fact, perhaps better to be 7.0.*|7.1.* heh?

[Ocramius]

@asgrim indeed, better choice. Updating now.