Updated minimum PHP to 1.7.3 in "Installation" (#729)
Updated minimum PHP to 1.7.3 in "Installation" (#729)

* Updated minimum PHP to 1.7.3 in "Installation"

* Wrap PHP version number in a version check.

getgrav/grav-learn,getgrav/grav-learn,getgrav/grav-learn,getgrav/grav-learn