Migrate dropwizard-spdy from NPN protocol to ALPN

Resolve #794

NPN is a deprecated protocol and ALPN it's successor.
Jetty doesn't support NPN for Java 8, so it makes sense to make a switch.

Look for reference:
http://www.eclipse.org/jetty/documentation/9.2.3.v20140905/alpn-chapter.html

I wasn't able to write a unit test (code is too coupled and starting a server requires alpn-boot in JVM's bootpath), so I just tested this PR on my local machine in the following configuration:

````
Browser client: 
Browser Chrome Version 39.0.2171.95 (64-bit)

Java client:
com.squareup.okhttp:okhttp:2.1.0 (little hacky, because they use spdy/3.1)

JRE7:
java version "1.7.0_67"
Java(TM) SE Runtime Environment (build 1.7.0_67-b01)
Java HotSpot(TM) Server VM (build 24.65-b04, mixed mode)

ALPN-boot:
alpn-boot-7.1.2.v20141202.jar

JRE8:
java version "1.8.0_25"
Java(TM) SE Runtime Environment (build 1.8.0_25-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.25-b02, mixed mode)

ALPN-boot:
8.1.2.v20141202
````