Require phpspec/prophecy as dev dependency

sebastianbergmann/phpunit#5033 changed phpunit
to no longer depend on phpspec/prophecy. This results in the need to add
a development dependency for phpspec/prophecy. Doing so results in a
warning, since PHPUnit\Framework\TestCase::prophesize() is deprecated
and will be removed in PHPUnit 10. While we could use the trait provided
by phpspec/prophecy-phpunit instead, it would break PHP < 7.3 support.

* Closes php-mock#50

composer.json

@@ -24,5 +24,8 @@
     },
     "archive": {
         "exclude": ["/tests"]
+    },
+    "require-dev": {
+        "phpspec/prophecy": "^1.15"
     }
 }