Join version constraints with a space

[fvictorio]

Before this change, the parser would take something like this:

pragma solidity >= 0.5.0 <0.7.0

and give the pragma value as >=0.5.0<0.7.0, which is an invalid range (at least to node-semver).

With this change, >=0.5.0 <0.7.0 will be returned instead. Notice though that the space after >= was lost.