Use an exact version of Solidity compiler

[oed]

eg
pragma solidity 0.4.4;
instead of
pragma solidity ^0.4.4;

Helps prevent compiling and thus testing and deploying with unintended or mismatching compiler versions (like tests done with one version, and deployment is with another version).

Also, some files have a different pragma from other files and all files should use the compiler version unless there's a good reason and clear documentation and explanation.