If you are able to use Julia (http://julialang.org/), you can use OpenGene (https://github.com/OpenGene/OpenGene.jl) to do this easily

Following code is tested:

using OpenGene

# no quality can be higher than K
const QUAL_LIMIT = 'K'
# how much you want to increase it
const QUAL_INC = 10

# R1.fq is the file name of your fastq
istream = fastq_open("R1.fq")
ostream = fastq_open("R1.out.fq","w")

# fastq_read will return an object FastqRead {name, sequence, strand, quality}
# fastq_write can write a FastqRead into a ouput stream
while (fq = fastq_read(istream))!=false
    qual_arr = [q for q in fq.quality.qual]
    for i in 1:length(qual_arr)
        qual_arr[i] = min(qual_arr[i]+QUAL_INC, QUAL_LIMIT)
    end
    fq.quality.qual = ASCIIString(qual_arr)
    fastq_write(ostream, fq)
end

close(ostream)

You can install Julia by sudo apt-get install julia if you are running Ubuntu, after you install it, just change the file names in the code, launch julia, paste the code and press ENTER.