Changes the Tachyon client version from 0.5 to 0.6 in spark core and distribution script.

New dependencies in Tachyon 0.6.0 include

commons-codec:commons-codec:jar:1.5:compile
io.netty:netty-all:jar:4.0.23.Final:compile

These are already in spark core.

Author: Calvin Jia <jia.calvin@gmail.com>

Closes #4867 from calvinjia/upgrade_tachyon_0.6.0 and squashes the following commits:

eed9230 [Calvin Jia] Update tachyon version to 0.6.1.
11907b3 [Calvin Jia] Use TachyonURI for tachyon paths instead of strings.
71bf441 [Calvin Jia] Upgrade Tachyon client version to 0.6.0.