Add libelf-dev to Ubuntu build deps

This was required on a Debian system, but in lieu of instructions for that distribution, and as Debian users will follow Ubuntu, would you consider verifying it on Ubuntu and accepting this PR?

INSTALL.md

@@ -150,7 +150,7 @@ sudo apt-get update
 
 # All versions
 sudo apt-get -y install bison build-essential cmake flex git libedit-dev \
-  libllvm3.7 llvm-3.7-dev libclang-3.7-dev python zlib1g-dev
+  libllvm3.7 llvm-3.7-dev libclang-3.7-dev python zlib1g-dev libelf-dev
 ```
 
 ### Install and compile BCC