Better (hopefully) detection of single exon transcripts.

The gem apparently does not draw the angled line representing the (only) intron of a (directed) transcript composed by two exons and no UTRs.
In fact, a terminal exon is removed from the list at glyph.rb:411 and the following exon count (line 434) is not correct anymore.

This commit changes the single exon test by checking the number of gaps instead of that of exons.