Dependency updates (issue #13)

Compile scope:
- com.squareup.okhttp3:okhttp .......... 3.12.1 -> 3.14.0
- org.yaml:snakeyaml ................... 1.23 -> 1.24

Test scope:
- org.assertj:assertj-core ............. 3.12.0 -> 3.12.2

Maven plugin updates:
- maven-javadoc-plugin ................. 3.0.1 -> 3.1.0

pom.xml

@@ -137,7 +137,7 @@
         <dependency>
             <groupId>org.yaml</groupId>
             <artifactId>snakeyaml</artifactId>
-            <version>1.23</version>
+            <version>1.24</version>
         </dependency>
         <dependency>
             <groupId>org.eclipse.jetty</groupId>
@@ -170,7 +170,7 @@
         <dependency>
             <groupId>org.assertj</groupId>
             <artifactId>assertj-core</artifactId>
-            <version>3.12.0</version>
+            <version>3.12.2</version>
             <scope>test</scope>
         </dependency>
         <dependency>
@@ -182,7 +182,7 @@
         <dependency>
             <groupId>com.squareup.okhttp3</groupId>
             <artifactId>okhttp</artifactId>
-            <version>3.12.1</version>
+            <version>3.14.0</version>
             <scope>test</scope>
         </dependency>
     </dependencies>
@@ -203,7 +203,7 @@
             </plugin>
             <plugin>
                 <artifactId>maven-javadoc-plugin</artifactId>
-                <version>3.0.1</version>
+                <version>3.1.0</version>
             </plugin>
             <plugin>
                 <artifactId>maven-compiler-plugin</artifactId>