fix a few dependence issues in install-mxnet-ubuntu-r.sh

[lazyparser]

No description provided.

[miguelgfierro]

Hi @lazyparser for R it's recommended to install MRO or MRS rather than the distribution that comes with Ubuntu, which is quite old.

[lazyparser]

Hi @miguelgfierro thank you for your recommendation :-). Although MRO is a good distro, the default packages are more easier for beginners to config and install. A better solution would be install r-base-core only when 'Rscript' is absent on user's environment. I'll update the patch.

[piiswrong]

@lazyparser Does this patch fix it for the default distro?
@miguelgfierro Does this patch break it for MRO or MRS? If not we should merge it

[miguelgfierro]

It shouldn't break

[thirdwing]

libcurl4-openssl-dev and libssl-dev are required by devtools.

@miguelgfierro You can add the PPA for latest R build if necessary.