Update composer.json to require bcmath 

I tried running GeoIP2 on a box without the BC Math PHP extension and got the error:

Fatal error: Call to undefined function MaxMind\Db\Reader\bcadd() in /home/vagrant/Akkroo/publish/vendor/maxmind-db/reader/src/MaxMind/Db/Reader/Decoder.php on line 252

The library should depend on ext-bcmath to ensure users do not run in to this problem.

Additionally, the package did not depend on a specific minimum version of PHP.  I assumed the same version as the GeoIP2 package.