Fix LEMONdBTest.test_add_and_get_pm_correction() (#42)

As in commit 2e3e099, the message of the sqlite3.IntegrityError
exception has changed between SQLite versions 3.7.13 and 3.8.7.1, as
seen in #42. Use in LEMONdBTest.test_add_and_get_pm_correction() a
regular expression that works with all versions of SQLite.