Revert "Add Emacs 26 pretest to Travis CI job"

This reverts commit 2e2dd75.

Fix handling of EPUBSTYLE

In org mode 9.0:

(org-split-string "") -> '()

In org mode 9.1+:

(org-split-string "") -> '("")

So we switch to split-string, which does what we want.

Fixes ofosos#15

Remove language name before source block

Properly expand directory names on MacOS

Fix xhtml export

ox-epub would under certain conditions generate invalid xhtml, this
change should fix these conditions.

Add condition-case handler around export process

This handler will remove the temporary directory and print a message,
should any error arise.

Push another rename fix

Merge pull request ofosos#1 from syohex/lib

Load missing library 'seq.el'

update the examples

remoe unused variable and change mapcar to mapc