Cannot install in org without Orders Enabled

[daveerickson]

When trying to install in a sandbox org without Orders enabled, throws exception:

ApexClass Invalid type: OrderItem
fflib_ApexMocksUtilsTest: Invalid type: OrderItem

This appears to be related to a test method added in this PR: #156

I am wondering is testing with Product2, OpportunityLineItem, and Opportunity could validate the same behavior?

[daveerickson]

PR that resolves this issue: #159