Update `<scala.binary.version>` prop in POM when switching between Scala 2.10/2.11

ScrapCodes for review. This `sed` command is supposed to just replace the first occurrence, but it replaces them all. Are you more of a `sed` wizard than I? It may be a GNU/BSD thing that is throwing me off. Really, just the first instance should be replaced, hence the `[WIP]`.

NB on OS X the original `sed` command here will create files like `pom.xml-e` through the source tree though it otherwise works. It's like `-e` is also the arg to `-i`. I couldn't get rid of that even with `-i""`. No biggie.

Author: Sean Owen <sowen@cloudera.com>

Closes #4876 from srowen/SPARK-5143 and squashes the following commits:

b060c44 [Sean Owen] Oops, fixed reversed version numbers!
e875d4a [Sean Owen] Add note about non-GNU sed; fix new pom.xml update to work as intended on GNU sed
703e1eb [Sean Owen] Update scala.binary.version prop in POM when switching between Scala 2.10/2.11