Fix failing test for 24 hr locales (#44)

This test was failing for me. I use en-US locale, but I set my clock to
24 hour. It's admittedly uncommon, but I'd wager this test is failing
for anyone in a non-12 hours locale.

To be honest, I expected the existing date format string `hh:mm a` to be
sufficient, but apparently its not.

MapboxNavigationTests/Sources/Tests/StyleManagerTests.swift

@@ -80,7 +80,8 @@ class StyleManagerTests: XCTestCase {
         let dateFormatter = DateFormatter()
         dateFormatter.dateFormat = "hh:mm a"
         dateFormatter.timeZone = TimeZone(identifier: "PST")
-
+        dateFormatter.locale = Locale(identifier: "en_US")
+
         NSTimeZone.default = NSTimeZone(abbreviation: "PST")! as TimeZone
 
         let beforeSunrise = dateFormatter.date(from: "05:00 AM")!