Add javax.activation dependency, close AsyncHttpClient#1541

Motivation:

javax.activation is available in JDK8 but module has to be enabled in
JDK9 when running with module path and is going away in JDK10.

Modification:

Add explicit dependency

Result:

AHC successful JDK9 build

Author: slandelle

pom.xml

@@ -300,6 +300,11 @@
       <artifactId>slf4j-api</artifactId>
       <version>${slf4j.version}</version>
     </dependency>
+    <dependency>
+      <groupId>com.sun.activation</groupId>
+      <artifactId>javax.activation</artifactId>
+      <version>${activation.version}</version>
+    </dependency>
     <!-- Test dependencies -->
     <dependency>
       <groupId>ch.qos.logback</groupId>
@@ -398,6 +403,7 @@
     <netty.version>4.1.24.Final</netty.version>
     <slf4j.version>1.7.25</slf4j.version>
     <reactive-streams.version>1.0.2</reactive-streams.version>
+    <activation.version>1.2.0</activation.version>
     <netty-reactive-streams.version>2.0.0</netty-reactive-streams.version>
     <rxjava.version>1.3.6</rxjava.version>
     <rxjava2.version>2.1.13</rxjava2.version>