Make locationManager initialization thread safe

Fixes #316

Also it turns out that this property is readonly, so I made it readonly.

Sources/MapboxMobileEvents/MMELocationManager.m

@@ -21,7 +21,7 @@
 @interface MMELocationManager () <CLLocationManagerDelegate>
 
 @property (nonatomic) id<MMEUIApplicationWrapper> application;
-@property (nonatomic) CLLocationManager *locationManager;
+@property (nonatomic, readonly) CLLocationManager *locationManager;
 @property (nonatomic, getter=isUpdatingLocation, readwrite) BOOL updatingLocation;
 @property (nonatomic) NSDate *backgroundLocationServiceTimeoutAllowedDate;
 @property (nonatomic) NSTimer *backgroundLocationServiceTimeoutTimer;
@@ -31,16 +31,15 @@ @interface MMELocationManager () <CLLocationManagerDelegate>
 
 @implementation MMELocationManager
 
-@synthesize locationManager = _locationManager;
-
 - (void)dealloc {
     _locationManager.delegate = nil;
 }
 
 - (CLLocationManager *)locationManager  {
-    if (_locationManager == nil) {
+    static dispatch_once_t onceToken;
+    dispatch_once(&onceToken, ^{
         _locationManager = [[MMEDependencyManager sharedManager] locationManagerInstance];
-    }
+    });
     return _locationManager;
 }
 
@@ -110,17 +109,6 @@ - (NSString *)accuracyAuthorizationString {
 }
 #endif
 
-- (void)setLocationManager:(CLLocationManager *)locationManager {
-    if (locationManager == nil) {
-        _locationManager = locationManager;
-    } else {
-        id<CLLocationManagerDelegate> delegate = _locationManager.delegate;
-        _locationManager.delegate = nil;
-        _locationManager = locationManager;
-        _locationManager.delegate = delegate;
-    }
-}
-
 - (void)stopMonitoringRegions {
     for(CLRegion *region in self.locationManager.monitoredRegions) {
         if([region.identifier isEqualToString:MMELocationManagerRegionIdentifier]) {