Update the manual's installation section

[isulitperalejo]

In the manual's installation section under prerequisites, the first command apt install build-essential llvm-6.0 clang-6.0 libclang-6.0-dev cmake libssl-dev pkg-config yields the following error:

Package clang-6.0 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Unable to locate package llvm-6.0
E: Couldn't find any package by glob 'llvm-6.0'
E: Package 'clang-6.0' has no installation candidate
E: Unable to locate package libclang-6.0-dev
E: Couldn't find any package by glob 'libclang-6.0-dev'

[kkysen]

Yeah, we should change this. LLVM 6.0 is very old by now.