Fixes compatibility with hapi 4.0 and joi 3.1.0.

hapi-swagger installed on hapi 4.0 server crashed due to
incompatibility between joi 3.0 and 3.1.
A validation object created in hapi-swagger with joi 3.0.x gets
evaluated by hapi with joi 3.1.x and server crashes with exception
"Alternatives require more than one".