Added OneOnly.mat test case data

* inst/extdata/OneOnly.mat: Added Matlab generated file that causes the
  following problem when reading:

  Error in read.mat("OneOnly.mat") :
  InflateDimensions: Reading dimensions expected type MAT_T_INT32
Add read OneOnly.mat test case

* tests/one_only.R: Added file with test case to read OneOnly.mat
Add .travis.yml