PDL core only update

[zmughal]

Update of #113.

@mohawk2, does this look right?

[zmughal]

Summary of changes relative to pdl-core-only:

• Fix conflicts in .gitignore .travis.yml MANIFEST MANIFEST.SKIP t/config.t
• Make sure that the .travis.yml stays containerised.
• Make sure the Changes dump of git log is not run in $preop (in Makefile.PL)
• Make sure PLplot stays out of the MANIFEST after it was dropped

Diff between branches:

diff \
    <(git log master..origin/pdl-core-only-update -p --cc  | grep -v '^\(commit\|index\|@@\)' ) \
    <(git log master..origin/pdl-core-only -p --cc         | grep -v '^\(commit\|index\|@@\)' ) | $PAGER

All the important files that can be inspected for changes:

( git log master..origin/pdl-core-only-update -p --cc | grep '^diff'; git log master..origin/pdl-core-only -p --cc | grep '^diff' ) \
    | sort -u | grep -v -e kitchen-sink -e Libtmp