travis: Lock asciidoctor to 1.5.4
travis: Lock asciidoctor to 1.5.4

1.5.5 (the newly released) seems to break.

mdhender/hugo,mitchchn/hugo,MarkDBlackwell/hugo,gohugoio/hugo,anthonyfok/hugo,gohugoio/hugo,mayah/hugo,mayah/hugo,Zenithar/hugo,digitalcraftsman/hugo,anthonyfok/hugo,anthonyfok/hugo,bogem/hugo,mitchchn/hugo,jygastaud/hugo,Zenithar/hugo,tubo028/hugo,jygastaud/hugo,mdhender/hugo,mdhender/hugo,mdhender/hugo,bogem/hugo,fedelibre/hugo,digitalcraftsman/hugo,MarkDBlackwell/hugo,gohugoio/hugo,tubo028/hugo,fedelibre/hugo,fedelibre/hugo,fedelibre/hugo