travis builds failing due to rstan compilation issues

My package builds on Linux are failing with the following compilation error for rstan package:

lto-wrapper: fatal error: make returned 2 exit status
compilation terminated.
/usr/bin/ld: lto-wrapper failed
collect2: error: ld returned 1 exit status
make: *** [rstan.so] Error 1
ERROR: compilation failed for package ‘rstan’

How can I modify my travis.yaml to successfully install rstan on Travis?

Build log: https://travis-ci.org/IndrajeetPatil/statsExpressions/builds/638404778?utm_source=github_status&utm_medium=notification

Travis yaml: https://github.com/IndrajeetPatil/statsExpressions/blob/master/.travis.yml