Define minimal supported version of PHP needed into composer.json

Address hoaproject/Central#70.

So force Travis to wait more time.

JSON grammar is $LL(k=0)$. This is good for performance to fix $k$ to
$0$, it avoids potential u...

UTF-8 is an issue with JSON, because we must handle surrogate pairs
([1], [2]). This patch imple...

We avoid using `\d` instead of `[0-9]` because `\d` _might_ match other
characters.

`{}` and `[]` are valid, even if empty.

We don't want skip tokens inside the strings. So now a string is defined
by a single token inste...

The [RFC4627] defines whitespace as follows (in Section 2. JSON Grammar):
> Insignificant whites...