Fixing Travis testSetup_Constants for version 1.5.2

This should resolve the following Travis CI build failure:

```text
Time: 0 seconds, Memory: 37.25Mb
There was 1 failure:
1) Easy_Digital_DownloadsTest::testSetup_Constants
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-1.5.2
+1.5.1
/home/travis/build/pippinsplugins/Easy-Digital-Downloads/travis/tests/easy-digital-downloadsTest.php:99
```