Install gsl package explicitely on Travis

Author: Alexandre Génin

.travis.yml

@@ -15,7 +15,7 @@ before_install:
 script:
   # Test first the fitting results against pli code
   - R -e "update.packages(ask = FALSE)"
-  - R -e "install.packages(c('testthat', 'roxygen2', 'devtools'))"
+  - R -e "install.packages(c('testthat', 'roxygen2', 'devtools', 'gsl'))"
   - R -e "devtools::install_deps(upgrade = 'always', dependencies = TRUE, repos = 'https://cloud.r-project.org')"
   - R -e "EXTENDED_TESTS <- TRUE; devtools::test(filter = 'psdfit')"
   - R -e "EXTENDED_TESTS <- TRUE; devtools::test(filter = 'xmin')"