Throw an error when minVersion map has not a string as a browser version

fixes #165

src/bowser.js

@@ -540,6 +540,10 @@
     for (var browser in minVersions) {
       if (minVersions.hasOwnProperty(browser)) {
         if (_bowser[browser]) {
+          if (typeof minVersions[browser] !== 'string') {
+            throw new Error('Browser version in the minVersion map should be a string: ' + browser + ': ' + String(minVersions));
+          }
+
           // browser version and min supported version.
           return compareVersions([version, minVersions[browser]]) < 0;
         }

test/test.js

@@ -127,6 +127,12 @@ describe('Unsupported browser check', function() {
     assert.equal(supported, false);
   });
 
+  it('should throw an error when minVersion map has a number, but not a string', function() {
+    assert.throws(() => {
+      browser.check({msie: 11}, this.ie10_6);
+    }, /Browser version in the minVersion map should be a string/);
+  });
+
   it('should be passed by #check for IE10.6 when version was not specified', function() {
     var supported = browser.check({}, this.ie10_6);
     assert.equal(supported, true);