expose EnsureDevtools again through old command devtools_install
expose EnsureDevtools again through old command devtools_install

Reason: Running extensive integration tests (such as in tikzDevice) that are not part of R CMD check but require installation of the package

Installation can be done easily by `devtools:::install('.')`

hadley/r-travis,robzhu/r-travis,qinwf/r-travis,eddelbuettel/r-travis,hadley/r-travis,qinwf/r-travis,robzhu/r-travis,eddelbuettel/r-travis,craigcitro/r-travis,craigcitro/r-travis