Replace FileInputStream with FileReader in TriCeraParameters

[woosh]

This PR replaces instantiation of a FileInputStream with an instantiation of a FileReader in the TriCeraParameters class. This is to adapt the TriCeraParameters class to recent changes in the Eldarica API, please see uuverifiers/eldarica#53 for details. Without the changes TriCera fails to build with the following error:

[error] /home/dev/repos/tricera/src/tricera/params/TriCeraParameters.scala:380:27: type mismatch;
[error]  found   : java.io.FileInputStream
[error]  required: java.io.Reader
[error]       fileName = fn; in = new FileInputStream(fileName); parseArgs(rest)
[error]

[zafer-esen]

Thanks for this PR!