Upgrade mocha package to its latest 5.* version

The 5.* version is the latest one that supports node.js 4. Upgrade to
latest 9.2.* version requires node.js 12.*

Improve several unit tests as they started to fail because of
mochajs/mocha#3226

package.json

@@ -28,7 +28,7 @@
   "devDependencies": {
     "coveralls": "^2.11.9",
     "istanbul": "^0.4.3",
-    "mocha": "2.x"
+    "mocha": "^5.2.0"
   },
   "dependencies": {
     "findhit-proxywrap": "^0.3.12",

test/smpp.js

@@ -116,31 +116,46 @@ describe('Session', function() {
 	describe('smpp.connect()', function() {
 		it('should use 2775 or 3550 as default port', function() {
 			var session = smpp.connect();
+			session.on('error', function() {});
 			assert.equal(session.options.port, 2775);
+
 			session = smpp.connect({tls: true});
+			session.on('error', function() {});
 			assert.equal(session.options.port, 3550);
+
 			session = smpp.connect('smpp://localhost');
+			session.on('error', function() {});
 			assert.equal(session.options.port, 2775);
+
 			session = smpp.connect('ssmpp://localhost');
+			session.on('error', function() {});
 			assert.equal(session.options.port, 3550);
 		});
 
 		it('should be backward compatible', function() {
 			var session = smpp.connect('127.0.0.1');
+			session.on('error', function() {});
 			assert.equal(session.options.port, 2775);
 			assert.equal(session.options.host, '127.0.0.1');
+
 			session = smpp.connect('127.0.0.1', 1234);
+			session.on('error', function() {});
 			assert.equal(session.options.port, 1234);
 			assert.equal(session.options.host, '127.0.0.1');
 		});
 
 		it('should properly parse connection url', function() {
 			var session = smpp.connect('smpp://127.0.0.1:1234');
+			session.on('error', function() {});
 			assert.equal(session.options.port, 1234);
 			assert.equal(session.options.host, '127.0.0.1');
+
 			session = smpp.connect('ssmpp://localhost');
+			session.on('error', function() {});
 			assert(session.options.tls);
+
 			session = smpp.connect({ url: 'ssmpp://127.0.0.1:1234'});
+			session.on('error', function() {});
 			assert(session.options.tls);
 			assert.equal(session.options.port, 1234);
 			assert.equal(session.options.host, '127.0.0.1');