fix jdk6 build

[xuwei-k]

Maven central and Bintray dropped TLS 1.1 support, require TSL 1.2. JDK6 does not support TLS 1.2.

• https://blog.sonatype.com/enhancing-ssl-security-and-http/2-support-for-central
• https://jfrog.com/knowledge-base/why-am-i-failing-to-work-with-jfrog-saas-service-with-tls-1-0-1-1/
• Find out what we can do to release for jdk6 instead of jdk7 scala-collection-compat#77

[ashawley]

TLS 1.2 works in Oracle JDK6, just not in openjdk6, see scala/sbt-scala-module#41

[xuwei-k]

use http instead of https? 🤔
scala/scala-xml#247