Log Message:
Update to 0.3.15

Upstream changes:
Changes in inline version 0.3.15 (2018-05-18)
Correct requireNamespace() call thanks (Alexander Grueneberg in #5).

Small simplification to .travis.yml; also switch to https.

Use seq_along instead of seq(along=...) (Watal M. Iwasaki) in #6).

Update package manual page using references to DESCRIPTION file [CRAN request].

Minor packaging updates.